The present invention relates to a piping system for supplying ultra-pure water to be used in large quantity in electronic industry such as semiconductor industry, or in biochemical and pharmaceutical industry.
In the past, a system as shown in FIG. 3 has been known as a piping system for supplying ultra-pure water for electronics industry, especially in the application for the manufacture of semiconductor integrated circuits.
According to such system, the primary pure water produced by a primary ultra-pure water producing system (not shown) is supplied to a circulation tank 102 through a primary pure water supply pipe 101. The primary water stored in the circulation tank 102 is pressurized by a pressure pump 103 and is purified through ultraviolet sterilizing unit (hereinafter referred as "UV sterilizing unit") 104, cartridge polishers 105, 106 and 107 and ultrafilter units 108 and 109. 122 represents a drain pipe for ultrafilter unit. The ultra-pure water thus purified is supplied to each of the user points 119, 120 and 121 through an outward pipe, connection pipes 113, 114 and 115 and branching valves 116, 117 and 118. After passing through connection pipes 113, 114 and 115, the ultra-pure water flows through a circulation route to return to the circulation tank 102 through a return pipe 112. The connection pipes 113, 114 and 115 are composed of upstream pipe sections 113-1, 114-1 and 115-1 and downstream pipe sections 113-2, 114-2 and 115-2.
FIG. 4 shows a piping system for supplying ultra-pure water of FIG. 3. 201 represents the entire ultra-pure water producing system. 131, 132 and 133 show the branching points of the connection pipes 113, 114 and 115 to the user points. The branching valves 116, 117 and 118 are integrally composed of the branching points 131, 132 and 133 respectively.
The ultra-pure water produced by such ultra-water producing unit is often supplied to the ultra-pure water using unit (hereinafter called as "user point", meaning the place where ultra-pure water is used). In a normal system, the number of the user points is extremely many, while FIG. 3 shows an example where 3 user points are provided.
However, in such piping system of conventional type, the quantity of ultra-pure water flowing in the outward pipe 110 is decreased when the number of the user points is increased, i.e. when the number of branching points is increased. On the contrary, the quantity of water in the return pipe 112 is increased when the number of the branching points is increased. Because the condition to use ultra-pure water at each user point is not constant, it has been difficult in the past to maintain the pressure between the outward pipe 110 and return pipe 112 of each of the connection pipes 113, 114 and 115 at constant level, and it has been impossible to supply a constant quantity of ultra-pure water to each of the user points at all times.
Also, in a conventional type piping system, the connection pipes 113, 114 and 115 have the same inner diameter upstream and downstream of the branching points. The branching valves 116, 117 and 118 are designed in such manner that, when they are opened, the pressure at the branching points 131, 132 and 133 is approximately equal to the atmospheric pressure. For example, in case the branching valve 117 is opened when ultra-pure water is used in the ultra-pure water using unit 120, the pressure at the branching point 132 is approximately equal to the atmospheric pressure.
On the other hand, the pressure of ultra-pure water flowing in the outward pipe 110 and the return pipe 112 is adjusted to the valve higher by 1-2 kg/cm.sup.2 than the atmospheric pressure in normal case. As the result, there occurs in the connection pipes 113, 114 and 115 the flow going from the outward pipe 110 to the branching point 132 and the flow going from the return pipe 112 to the branching point 132, and these flows are mixed and supplied to the ultra-pure water using unit. The ultra-pure water flowing from the return pipe 112 to the branching pipe 132 is the ultra-pure water, which has passed through the other connection pipes 113 and 115 and the return pipe 112. Thus, it contains more impurities than the ultra-pure water, which has flown through only the upstream pipe section 114-1 of the connection pipe from the outward pipe 110 because of the elution of impurities from piping materials.
For example, when water quality is expressed by electric resistivity, which is an index of the concentration of ionic impurities contained in ultra-pure water, the resistivity of ultra-pure water supplied to the ultra-pure using unit 120 through only the upstream pipe section 114-1 of the connection pipe from the outward pipe 110 is 18.2 M.OMEGA..cm. In contrast, the resistivity of the water to be supplied to the ultra-pure water using unit 120 through the downstream pipe section 114-2 of connection pipe from the return pipe is 18.0 M.OMEGA..cm. This difference in resistivity 0.2 M.OMEGA..cm corresponds to the difference of concentration of about 0.2 ppb. In other words, there is a limitation in supplying ultra-pure water with ultra-purity in the conventional type piping system for supplying ultra-pure water.
The object of the present invention is to offer a piping system, which can supply a constant quantity of ultra-pure water to each of the user points at all times and can provide stable supply of ultra-pure water with ultra-high putity by preventing the counterflow from the return pipe to the ultra-pure water using unit.
The invention comprises a circulation tank to store primary pure water from a primary pure water producing unit, a pump for sending the primary pure water from said circulation tank, an outward pipe , one end of which is connected to a final purifying unit to purify primary pure water from said pump to ultra-pure water, a plurality of connection pipes, each end of which is connected to the other end of said outward pipe, a branching pipe connected between the middle of said connection pipe and the ultra-pure water using unit and having a branching valve to adjust the water quantity, and a return pipe connected between the other end of said connection pipe and said circulation tank, characterized in that means for controlling the output of said pump is provided to keep the water pressure at constant level by detecting the water pressure in said outward pipe.
The invention describes a piping system wherein the energy loss when water is passed upstream of the branching pipe of said connection pipe is equal to the energy loss when water is passed downstream of said branching pipe.
The invention describes a piping system wherein said connection pipe has larger pipe diameter upstream than downstream.
The invention describes a piping system wherein said branching valve can limit the flow rate so that the pressure upstream is higher than the pressure downstream.
The invention describes a piping system wherein there are two or more combinations of said outward pipe and return pipe between said final purifying unit and the ultra-pure water using unit.
The invention describes a piping system for supplying ultra-pure water wherein said connection pipe is contained partially or totally in said ultra-pure water using unit.
The invention is a piping system wherein a plurality of said branching pipes are furnished on said connection pipe.
The invention offers a piping system wherein the pipe diameter of said connection pipe is gradually reduced according to the arrangement order of said branching pipe arranged from said outward pipe to said return pipe.
The invention offers a piping system wherein said branching valve is furnished at the branching portion between said connection pipe and said outward pipe and at the connection between said connection pipe and said return pipe.
The invention offers a piping system for supplying ultra-pure water wherein said outward pipe, said return pipe and said connection pipe are made of high purity vinyl chloride resin, vinylidene fluoride resin or polyether-ether ketone.
The invention is a piping system wherein said outward pipe, said return pipe and said connection pipe are made of stainless steel having inner surface, on which passive state film made primarily of CR.sub.2 O.sub.3 is formed.
The pressure of water flow from pump is controlled by control means, and the pressure in the outward pipe and the return pipe is maintained at constant level. The energy loss when water passes through each of the connection pipes is equal to each other, and a constant quantity of ultra-pure water can be supplied to each user point through connection pipes.
The purity of ultra-pure water can be extensively improved when high purity vinyl chloride resin, vinylidene fluoride, vinylidene fluoride resin or polyether-ether ketone are selected as piping material and counterflow is prevented by setting the pipe diameter in upstream of the branching of connection pipe larger than that of upstream.